Some ophiacodonts were semi-aquatic, and few were fully aquatic, but some were fully terrestrial like Archaeothyris.[citation needed] Several ophiacodontids resembled small lizards, while others were larger with elongated skulls and massive shoulder girdles, probably to provide muscle attachment to support the weight of the large head.

Traditionally, Archaeothyris, Ophiacodon, Varanosaurus and the briefly described Baldwinonus, Clepsydrops, Echinerpeton, Stereophallodon and Stereorhachis are included in the Ophiacodontidae. Protoclepsydrops was also regarded as ophiacodontid, however there is debate as to whether or not it was a synapsid. Echinerpeton and Sterophallodon were included for the first time in a phylogenetic analysis by Benson (in press). Echinerpeton was found to be a wildcard taxon due to its small amount of known materials. It occupies three possible positions, falling either as the most basal synapsid, as the sister taxon of Caseasauria + more derived taxa, or as an ophiacodontid more derived than Archaeothyris. Below is a cladogram modified from the analysis of Benson (in press), after the exclusion of Echinerpeton:[1]